This invention relates to an aircraft skin lap splice between adjacent panels on an aircraft, and is particularly directed to the provision of an improved fuselage skin lap splice.
In the longitudinal fuselage skin lap joint splicing of adjacent panels on an aircraft, longitudinal rows of fasteners are used to connect the overlapping outer and inner skins. However, high load transfer occurs through the outermost rows of fasteners in such skin lap splices, resulting in high shear loads on such fasteners, and a limited fatigue life of the connection.
To overcome this problem, one or more so-called "finger doublers" are sometimes used at the skin lap splice between adjacent skin panels. One form of such prior art longitudinal skin lap splice arrangement is shown in FIG. 1 of the accompanying drawing. In this figure, numeral 10 is a longitudinal fuselage skin lap splice formed of the overlapping edge portions of an outer skin 12 and an inner skin 14 combined with a pair of finger doublers 16 and 18. The two full rows of rivets 20 and 22 are located in the overlap between the adjacent outer and inner skin portions, and the finger doublers 16 and 18 are added to decrease the critical bearing stress that would otherwise develop on such rows of rivets. The need for reducing the bearing stress exerted by the rivets on the skins is particularly pronounced for the outer skin 12 whenever the rivets are countersunk, or flush, since the parallel shank of the rivet is far more effective than the sloping head in transmitting load. Even though the rivet loads are common to both skins, the inner skin 14 is less critical because the full thickness is available to withstand the bearing loads on each rivet.
The finger doubler 16 at the top picks up one of the full rows of rivets 20 between the outer and inner skins, along the row of holes at 24 and the row of rivets at 26 which pass at 28 through the fingers 30 of the doubler 16. The lower finger doubler 18 fits inside the inner skin 14 and picks up all of the fasteners in rows 20 and 22 in holes 32 and 34, and also picks up all of the fasteners in row 36 of the inner skin 14, in the row of holes 38 of the fingers 40. Rows 26 and 36 contain holes spaced twice as far apart as in rows 20 and 22. The longeron 42, which functions to stabilize the splice or connection against buckling, is connected to the skins 12 and 14 and doubler 18 by the line of fasteners through the row of holes 34 in the inner skin, matching row 22 in the outer skin.
The term "fastener" is sometimes used generically to refer to all fasteners including, for example, bolts, rivets and pin-collar combinations. Aircraft joints can contain a mixture of threaded fasteners and rivets at different locations within the same joint, depending on the local load transfer. In the present invention the word fastener is used in its generic sense, and the word rivet is used to illustrate specific examples.
A simpler but less durable form of prior art lap splice is shown in FIG. 2, consisting of three (or sometimes two or four) full rows of fasteners 11, 13 and 15 through a uniformly wide overlap. The critical location is at the countersunk holes around the rivets 11 in the outer skin 17 nearest to the end of the inner skin 19. The holes at the opposite end of the splice are less critical because the continuous (inner) skin is not countersunk there. The full thickness of the skin is available to react the bearing stress due to the loads on fasteners 15 and to transmit the "bypass" load to the other fastener rows. Although the fastener loads peak at both ends of the overlap in FIG. 2, the holes adjacent to the end of the outer skin are less critical than at the opposite end because the load in the outer skin has been decreased by the loads transmitted by the other rows of fasteners. For the same reason, the inner skin 19 is more critical at the fastener row 15 than it is at row 11. The fatigue life of such a simple splice has sometimes been increased by using a row of protruding-head fasteners as shown at 11' in FIG. 2a for only the most critical row, in an otherwise flush joint. This local modification improves the fatigue life of the joint by decreasing the bearing stress in the outer skin 17 by having the entire thickness of the skin available to resist the bearing loads transmitted by fasteners 11', in contrast with only the parallel shank depth in FIG. 2. The difference is the depth of the countersink in the row of holes for the fasteners 11 in FIG. 2. The modified joint in FIG. 2a is now equally critical in the outer skin 17 at the row of fasteners 11' and in the inner skin at the row of fasteners 15 (see FIG. 2). While the fatigue life has been increased with respect to the flush joint shown in FIG. 2, the 50--50 likelihood of the cracks occurring where they would be visible from the outside and where they would not is a significant disadvantage. Also, the protruding head fasteners 11' in FIG. 2a will add to the aerodynamic drag of the airframe.
The overlap splices shown in FIGS. 1 and 2 have an eccentricity in load path between the inner and outer skins. This causes them to bend out of plane as the load is applied. An alternative prior art splice design to eliminate this bending is shown in the butt splice of FIG. 3, in which the adjacent skin panels 21 and 23 are butted together at 25 and spliced by the flange of the longeron 27, the external longitudinal strap 29, and the internal fingered splice plate 31. Since there is no direct connection between the skins, the load is transferred from one skin to the splice straps and back into the other skin at the opposite end of the joint. This design enables the skins to be safely operated at higher stress levels than with the lap splices shown in FIGS. 1 and 2, but requires almost twice as many fasteners. This design also uses fingers on the thin inner splice plate 31 both to reduce the fastener loads at the outermost full rows of fasteners and to increase the inspection intervals by permitting longer cracks to grow between the more widely spaced partial outermost rows.
It is an object of the present invention to provide an improved skin lap splice for aircraft.
Another object is to provide an improved fuselage skin lap splice for aircraft with respect to the prior art splice shown in FIG. 2, by reducing the load transfer on the most critical row of fasteners connecting the outer and inner skins of the splice, and thereby reducing metal fatigue.
A still further object is to provide an improved fuselage skin lap splice which is simpler in construction and formed of fewer components than the prior art lap splice of FIG. 1, and requiring fewer fasteners than the prior art butt splice of FIG. 3, while providing the same advantages from the finger doublers, namely reduced load transfer and bearing stress at the most critical fastener holes in the skins at the splice.